Stereo Preamplifier

K8060 Velleman Kit

When Maplin closed down in the UK  I was fortunate enough to pick up a couple of K8060 discrete power amplifier kits from Velleman.

They had been recommended to me by a work colleague.

It was a good lunchtime project to solder these up

K8060 kit from Maplin

over a a couple of months and the design had some interesting features like how the drive transistors are connected to header pins and down the the heat sink. I tested these on some Lab power supplies at work and they seemed to work quite well as amplifiers so they remained in the projects cupboard for quite along while until I could get back to them.

 

 

The Kit Build

The kit build was quite straightforward and the

Insert Resistors and diodes into K8060 PCB

instructions where excellent the first step was to insert all the resistors as shown in the picture opposite.

 

 

 

 

Insert diodes and headers into the K8060 PCB

Next step was to add the rectifier diodes and the header for the transistors onto the PCB. The rest of the components the rectifier capacitors PCB transistors and variable resistors are soldered onto the PCB.

 

K8060 Kit build complete

The board is then bolted to the heat sink and the output transistors are bolted down to the heatsink and the legs bent up in between the headers and soldered. So I built up two of these modules.

 

As I wanted to be able to run these directly from an iphone I needed to build a stereo preamplifier

The Preamplifier

The preamplifier is based on the TL072 dual amplifier in a non inverting amplifier configuration.

Stereo Preamplifier Schematic

The amplifiers are biased to half rail via R1 & R2 and mid rail is stabilized using C2. Audio is fed via C5 and C6 to the non-inveting amplifiers and amplified R5,R3 and R6 & R4, these values can be chosen to suit the level of gain required.

Pre-Amplifier Scope plot

Because the K8060 amplifier is powered from an external transformer  with an output higher than the amplifiers can stand it is necessary to add a discrete linear regulator formed around Q1 to protect the amplifier. Depending on the input voltage this transistor may need additional heat sinking. Diode D1 prevents reverse polarity connections.

The board is a dual sided PCB, and the bill of materials is listed below

Stereo Pre-amplifier Circuit
QTY  PART-REFS           VALUE               
---  ---------           -----               
Resistors
---------
2    R1,R2               22k                 
2    R3,R4               68k                 
2    R5,R6               10k                 
2    R7,R8               1k                  

Capacitors
----------
1    C1                  100n                
2    C2,C7               10uF                
6    C3-C6,C8,C9         1uF                 

Integrated Circuits
-------------------
1    U1                  TL072               

Transistors
-----------
1    Q1                  BC547BP             

Diodes
------
1    D1                  1N4148              
1    D2                  1N4749A             

Miscellaneous
-------------
1    J1                  TBLOCK-I2           
2    J2,J3               CONN-SIL2           
3    J4-J6               CONN-SIL3